Mold for steel casting



United States Patent 3,254,378 MOLD FGR STEEL CASTING Edwin J. Richard, Allentown, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Filed Sept. 10, 1964, Ser. No. 395,579

. 4 Claims. (Cl. 22-129) This invention relates broadly to molds for use in the casting of metal, and more particularly to sand molds ideally adapted for use in the production of large steel castings.

The primary requirement for sand molds use-d in the casting of steel is that said molds resist penetration of the molten steel. In addition, the metal-contacting surfaces of the mold should be formed of a material which resists chemical attack by the molten steel.

Prior to this invention, it was found that, in the case of the common mold sands such as silica and zircon, for example, the resistance to metal penetration varied inversely with the permeability of the sand. Sand molds used in the production of large steel castings were therefore formed of silica and Zircon sands of low permeability.

While the use of silica and zircon sands having low permeability decreased the degree of metal penetration, sufficient metal penetrated the mold so that the surfaces of the casting were of relatively poor quality. An extensive cleaning operation was therefore necessary. Furthermore, theme of sand of low permeability caused venting problems, expansion defects, and inhibited drying and curing of the mold. Also, air-setting oil-oxygen binders and air-curing resin binders could not readily be used with such sands, as the low permeability of the sand substantially lengthened. setting times, and the large quantity of binder required to be mixed with these sands resulted in excessive evolved gases and added to the cost. Consequently, clay, cereal, and oil mixtures were generally used as binders. Sand held together with these mixtures had to be hand-rammed, and molds so made had to be heavily reinforced with iron. Considerable care had to be exercised when handling these molds.

It is an object of this invention to providea sand mold having high resistance to penetration of molten steel.

It is a further object to provide a sand mold, the surfaces of which are resistant to chemical attack by molten steel.

Another object is to provide a sand mold having relatively high permeability and good dimensional stability.

I have discovered that the foregoing objects can be attained by providing a mold comprising a mixture of coarse chromite sand and a binder of the class consisting of airsetting oil-oxygen binders and air-curing resin binders, the metal-contacting surfaces of said mold being coated with a zircon wash.

Broadly, this invention is based upon the discovery that a mixture of chromite sand and a binder of the abovementioned class rapidly dissipates heat. Thus, molten steel in molds formed of this mixture quickly solidifies adjacent to the mold surfaces. This solid layer of steel forms before metal can penetrate into the mold, and provides a protective barrier against any metal penetration.

I have further discovered that the resistance to metal penetration is substantially independent of the permeability of the chromite sand used in the sand-binder mixture. Thus, molds may be formed of coarse grained chromite, and the above-described problems presented by the use of fine-grained sand are no longer encountered.

Molten steel readily reacts with the chromite sandbinder mixtures. The products of this reaction are difficult to remove from the surfaces of the casting. I have found that chemical reactions betw'een the chromite 3,254,378 Patented June 7, 1966 ice by applying a heavy coating of zircon wash to the metalcontacting surfaces of the mold. v

The chromite sand used in the sand-binder mix shouldbe relatively coarse. The permeability of the sand, as determined by the standard A.F.S. permeability test, should be at least 40, and preferably considerably higher, e.g. to 80. The sand is mixed with a binder of the class consisting of air-setting oil-oxygen binders and air-curing resin binders. The air-setting oil-oxygen binders are well known in the art and comprise a mixture of a metallic drying oil and other oils such as linseed, sunflower, tung, rape seed, and fish oil. Preferably, the binder contains an accelerator such as sodium perborate, although other perborates, as well as carbonates, peroxides, and chlorites, may be used.

The air-curing resin binders are also well known in the art, and comprise furfuryl alcohol resin, urea resin, and combinations thereof catalyzed with an acid such as an phosphoric acid solution.

The chromite sand-binder mix is formed around a pattern in the usual manner. Where a backing sand, e.g. silica, is used, the layer of chromite sand-binder mix must be sufiiciently thick so that the heat from the molten steel can readily be dissipated. Otherwise, there will be considerable metal penetration. In castings having sections of 20 inches or more, a layer of at least 4 inches of the chromite-binder mix is necessary.

After the mold has set, the metal-contacting surfaces thereof are provided with a heavy coating of a zircon wash such as zircon flour, water, a suspending agent, and abinder. 7

If the sand-binder is of the oil-oxygen type, the mold is then baked in the usual manner.

To demonstrate the superior penetration resistance of molds formed of coarse chromite sand and an air-setting or air-curing binder, a mold containing four standard 2" x 2" diameter cores was made. Two of the cores were made of zircon sand, one was made of silica sand, and one was made of chromite sand. The. sand for all four cores was mixed with an air-setting binder. No wash was applied.

The mold was bottom-gated and a steel was cast therein. The composition of the steel was:

Percent (wt.) C 0.89 Mn 0.65 P 0.009 S 0.005 Si 0.33 Fe Balance Table I Type of Sand Permeability Percent Penetration 1. Zircon 22.7 7.6 2. Zircon 38 17. 5 3. Silica... 25.8 11.9 4. Chromite 76 0. 4

It is apparent from Table I that the core containing coarse chromite sand had much 'better resistance to metal )enetration than the cores containing finer zircon and silica :and.

As a specific example of my invention, a core as- :embly mold was made to cast a large valve rotor. The iouring Weight of the casting was 125,000 pounds and the :asting shipped at 67,000 pounds.

The cores used in this core assembly mold were lined vith approximately 4" of chromite sand at the metal- :ontacting surfaces and backed with silica sand.

The sand mixes used for the facing and backing sand avere as follows.

Facing sand:

1000 pounds of dried chromite sand (crushed chromite ore) A.F.S. #72 pounds of Kold Set (oil-oxygen binder) 60 ounces of sodium perborate Backing sand:

1000 pounds of washed and dried silica sand A.F.S.

#93 pounds of Kold Set (oil-oxygen binder) 60 ounces of sodium perborate After the cores were air set and stripped from the core aoxes, the surfaces of the cores which the metal was to :ontact were washed with two coats of a zircon mold coating comprising 100 pounds of a 325 mesh zircon flour, pounds of a mold wash compound containing a suspending agent and binder, and enough water to develop a slurry of 77 Baum. The cores were baked at 400 F. for l8 hours.

After the mold was assembled, a portable dryer was placed on the mold, and the mold was dried for 24 hours at 200 F.

The metal poured was steel of the following analysis: .26 C, .54 Mn, .010 P, .016 S, .35 Si, .08 Ni, .10 Cr, .02 Mo, .08 Cu, .01 Sn, .634 Al, balance iron. The pouring temperature was 2720 F. and the steel was poured from a bottom pouring ladle using a 3 /2 nozzle. The casting was allowed to cool for five days.

After lifting the casting from the mold, the casting was hydroblasted and examined. There was no metal penetration and the sand peeled from the casting Without difficulty. The surface of the casting was very good, showing such detail as the brush marks left from applying the wash.

The molds of the invention have better resistance to metal penetration than prior molds. In addition, the molds resist attack by molten steel, have high permeability, and have good dimensional stability.

By reason of the small amount of metal penetration, as well as the virtual absence of a burnt-on coating on the casting, the time required to clean the surface of the casting is relatively short. In addition, the castings can be made with molds having closer tolerances than was heretofore possible.

I claim:

1. A mold for molten steel, said mold comprising a mixture of coarse chromite sand and a binder of the class consisting of air-setting oil-oxygen binders and air-curing backing of silica sand, a facing comprising a mixture of coarse chromite sand and a binder of the class consisting of air-setting oil-oxygen binders and air-curing resin binders, the metal-contacting surfaces of said mold being coated with a zircon wash.

3. A mold for molten steel, said mold comprising a backing of silica sand, a facing comprising a mixture of coarse chromite sand and a binder of the class consisting of air-setting oil-oxygen binders and air-curing resin binders, said facing being of a thickness sufficient to cause the steel adjacent the surfaces of the mold to solidify before there is any metal penetration, the metal-contacting surfaces of the mold being coated with a zircon wash.

4. A mold for molten steel, said mold comprising a mixture of chromite sand having an A.F.S. permeability. of at least 40 and a binder of the class consisting of air-setting oil-oxygen binders and air-curing resin binders, the metal contacting surfaces of said mold being coated with a zircon wash.

References Cited by the Examiner UNITED STATES PATENTS 2,282,349 5/ 1942 Wellings 22192 2,792,311 5/1957 Davies 264-30 3,122,801 3/1964 Merrefield 22129 J. SPENCER OVERHOLSER, Primary Examiner.

EUGENE MAR, Assistant Examiner. 

1. A MOLD FOR MOLTEN STEEL, SAID MOLD COMPRISING A MIXTURE OF COARSE CHROMITE SAND AND A BINDER OF THE CLASS CONSISTING OF AIR-SETTING OIL-OXYGEN BINDERS AND AIR-CURING RESIN BINDERS, THE METAL-CONTACTING SURFACES OF SAID MOLD BEING COATED WITH A ZIRCON WASH. 